Superconductivity of non-stoichiometric intermetallic compound NbB2

We report the synthesis, magnetic susceptibility and crystal structure analysis for NbB2+x(x=0.0–1.0) samples. The study helps find a correlation among the lattice parameters, chemical composition and the superconducting transition temperature TcTc. Rietveld analysis is done on the XX-ray diffraction patterns of all synthesized samples to determine the lattice parameters. The aa parameter decreases slightly and has a random variation with increasing xx, while cc parameter increases from 3.26 for pure NbB2 to 3.32 for x=0.4x=0.4 i.e. NbB2.4. With higher Boron content (x>0.4)(x>0.4) the cc parameter decreases slightly. The stretching of lattice in cc direction induces superconductivity in the non-stoichiometric niobium boride. Pure NbB2 is non-superconductor while the other NbB2+x(x>0.0) samples show diamagnetic signal in the temperature range 8.9–11 K. Magnetization measurements (M–HM–H) at a fixed temperature of 5 K are also carried out in both increasing and decreasing directions of field. The estimated lower and upper critical fields (Hc1Hc1 & Hc2Hc2) as viewed from M–HM–H plots are around 590 and 2000 Oe, respectively, for NbB2.6 samples. In our case, superconductivity is achieved in NbB2 by varying the Nb/B ratios, rather than changing the processing conditions as reported by others.